1. The Field of the Invention
The present invention relates to a method an apparatus to determine the quality of steam by measuring the velocity of steam flowing through the throat of a nozzle under critical flow conditions.
2. The Prior Art
Steam flooding has become an accepted practice for recovery of petroleum products from marginal fields or reservoirs that require a degree of stimulation to produce a satisfactory flow of crude petroleum. There is a need for a simple method and apparatus to determine the quality of steam at the wellhead of an injection well and at any location of the steam distribution network. The wellhead measurement, if simplified, would be particularly useful in determining the amount of heat which is applied to the underground reservoir by the injected steam.
The measurement or monitoring of steam quality is important since the steam's quality, and thereby its ability to heat up the reservoir or formation, affects the resulting production operations. Further, the quality of the steam which can be most economically injected into a particular substrate or reservoir is contingent on a number of circumstances. These include the age of the reservoir and the anticipated prospects for extracting commercially justified amounts of hydrocarbon products therefrom.
In brief, it is desirable that the quality of steam which is injected into each injection well be altered or adjusted to a level of quality that best conforms to the condition of the formation being produced by that well. Clearly the quality of the steam must be known before any alteration or adjustment can be made.
It is known that in order to be particularly effective in this type of stimulation operation, the flow of injected steam must be monitored by use of metering means positioned in the steam-carrying line adjacent the wellhead. It can be appreciated that steam will normally leave the steam generator or source at a known quality, pressure and mass flow rate. As the pressurized steam flow progresses towards an injection well, however, the quality will usually be substantially decreased. A decrease in the quality can be based on such factors as the distance between the well and the source and the effectiveness of pipe insulation. It will further depend on the pipe layout including number and orientation of fittings through which the steam has to travel prior to reaching the injection port or well because of phase separation that can occur in these fittings.
U.S. Pat. No. 4,836,032 discloses the use of an orifice plate in series with a critical flow choke to provide a method of measurement for both steam quality and mass flow rate. Either the orifice plate or the choke alone can be used to measure steam quality and mass flow rate. The invention described in this patent application is to measure only the steam quality.
U.S. Pat. No. 4,324,143 discloses a flowmeter to measure the flow rate of air at low pressure and low flow rate for automotive internal combustion engines. Although this invention involves a venturi it is primarily focused on sets of swirling vanes of specific design aimed to obtain an improved metering of the flow rate for air at low pressure and low velocity condition. The present invention is distinguished from this prior art in that it is used to determine the quality of steam by measuring its critical or sonic velocity at the throat to a nozzle.
U.S. Pat. No. 2,060,848 discloses an improved air velocity meter for measuring air or gas at very low velocities. One way to use this invention is to connect the meter to a probe which consists of two tubes that can be inserted through a hole on the side of the main air or gas duct. However, this invention is primarily concerned with the specific features of the air velocity meter.
It should be emphasized that the present invention is concerned with the determination of the steam quality by measuring the critical velocity of the steam at the throat of a nozzle. The usefulness of the present invention is for steam which is usually at a high pressures ranging from several hundreds to thousands psia. A unique feature of the present invention is based on the fact that the critical velocity of steam is a function of steam quality. The critical velocity of steam increases as the steam quality is increased. The magnitude of velocity is the highest velocity the steam can achieved under the prescribed condition. It can be from several hundred to more than one thousand feet per second. Clearly, the objective of the measurement, the type of fluid, the range of pressure and velocity are distinctly differed from those described in the above mentioned patents.